Magnetic media are widely used in various applications, particularly in the computer industry, and efforts are continually made with the aim of increasing the areal recording density, i.e., bit density of the magnetic media. Conventional thin-film type magnetic recording media, wherein a fine-grained polycrystalline magnetic alloy serves as the active recording layer, are generally classified as “longitudinal” or “perpendicular”, depending upon the orientation of the magnetic domains of magnetic material. In perpendicular magnetic recording media, residual magnetization is formed in a direction perpendicular to the surface of the magnetic medium, typically a layer of a magnetic material on a suitable substrate. Very high linear recording densities are obtainable by utilizing a “single-pole” magnetic transducer or “head” with such perpendicular magnetic media.
It is well-known that efficient, high bit density recording utilizing a perpendicular magnetic medium requires interposition of a relatively thick (i.e., as compared to the magnetic recording layer), magnetically “soft” underlayer (SUL) or “keeper” layer, i.e., a magnetic layer having a relatively low coercivity below about 1 kOe, between a non-magnetic substrate and a “hard” magnetic recording layer having perpendicular anisotropy K⊥ and a relatively high coercivity Hc of several kOe, typically about 3-6 kOe. The magnetically soft underlayer (SUL) e.g., of a NiFe alloy such as Permalloy serves to guide magnetic flux emanating from the head through the hard, perpendicular magnetic recording layer, typically comprised of a Co-based alloy material, such as CoCr. In addition, the magnetically soft underlayer (SUL) reduces susceptibility of the medium to thermally-activated magnetization reversal by reducing the demagnetizing fields which lower the energy barrier that maintains the current state of magnetization.
Referring to FIG. 1, a typical conventional perpendicular recording system 10 comprises a vertically oriented (i.e., perpendicular) magnetic medium 1 and a single-pole head 2. Medium 1 includes substrate 3, relatively thick soft magnetic underlayer (SUL) 4, at least one relatively thin non-magnetic (i.e., non-ferromagnetic) interlayer 5 (sometimes referred to as an “intermediate” layer), at least one relatively thin magnetically hard recording layer 6, and a thin protective overcoat layer 7. Interlayer 5 serves to: (1) prevent magnetic interaction between the SUL 4 and the recording layer 6 and (2) promote desired microstructural and magnetic properties of the magnetically hard recording layer 6.
As illustrated in FIG. 1, single-pole head 2 includes a main pole 8 and an auxiliary pole 9. As shown by the arrows in the figure indicating the path of the magnetic flux φ, flux φ is seen as emanating from main pole 8 of single-pole magnetic transducer head 2, entering and passing through vertically oriented, hard magnetic recording layer 6 in the region below main pole 8, entering and traveling along SUL 4 for a distance, and then exiting therefrom and passing through the perpendicular hard magnetic recording layer 6 in the region below auxiliary pole 9 of single-pole magnetic transducer head 2. The direction of movement of perpendicular magnetic medium 1 past transducer head 6 in the x-direction is indicated in the figure by the arrow above medium 1.
Perpendicular magnetic recording systems such as system 10 comprising perpendicular recording medium 1 include SUL 4 in order to channel the magnetic field from the main pole 8 of the single-pole head 2 and thereby increase the effective magnetic field applied to the magnetically hard recording layer 6. The increased magnetic field enables an increase in the media coercivity Hc which can be utilized, ultimately resulting in improvements in the media signal-to-noise ratio (SNR), thermal stability, and areal recording density.
With continued reference to FIG. 1, substrate 3 is typically disk-shaped and comprised of a non-magnetic metal or alloy, e.g., an Al-based alloy, such as Al—Mg having an Ni—P plating layer on the deposition surface thereof, or substrate 3 is comprised of a suitable glass, ceramic, glass-ceramic, polymeric material, or a composite or laminate of these materials. The relatively thick SUL 4 is typically comprised of an about 40-400 nm layer of a soft magnetic material selected from the group consisting of Ni, NiFe (Permalloy), Co, CoZr, CoZrCr, CoZrNb, CoFe, Fe, FeN, FeSiAl, FeSiAlN, FeCoC, FeCoB, etc. Relatively thin interlayer 5 typically comprises an up to about 30 nm thick layer of a non-magnetic material, such as TiCr. Magnetically hard recording layer 6 is typically comprised of an about 10 to about 25 nm thick layer of a Co-based alloy including one or more elements selected from the group consisting of Cr, Fe, Ta, Ni, Mo, Pt, V, Nb, Ge, B, and Pd, iron nitrides or oxides, or a (CoX/Pd or Pt)n multilayer magnetic superlattice structure, where n is an integer from about 10 to about 25, each of the alternating, thin layers of Co-based magnetic alloy is from about 2 to about 3.5 Å thick, X is an element selected from the group consisting of Cr, Ta, B, Mo, Pt, W, and Fe, and each of the alternating thin, non-magnetic layers of Pd or Pt is up to about 10 Å thick. Each type of hard magnetic recording layer material has perpendicular anisotropy arising from magneto-crystalline anisotropy (1st type) and/or interfacial anisotropy (2nd type).
Completing medium 1 is a protective overcoat layer 7, such as a layer of a diamond-like carbon (DLC) formed over magnetic recording layer 6, and a lubricant topcoat layer (not shown in the figure for illustrative simplicity), e.g., a layer of a perfloropolyether material, formed over the protective overcoat layer 7.
As indicated above, a conventionally-configured perpendicular magnetic recording medium such as illustrated in FIG. 1 typically comprises a relatively thick SUL 4 of a high magnetization (Ms) material (such as those enumerated supra) which exhibits in-plane anisotropy dominated by shape anisotropy 4πMs. The magnetic structure of the SUL 4 can contribute to degradation of media performance, at least for the following reason. Magnetic domain walls in the soft magnetic materials, whether such materials are present in the media as a SUL or in read/write transducer head(s), must be carefully controlled in order to avoid spurious system behavior. For example, the fringe magnetic field from a domain wall in the SUL can produce a large signal when it passes by the read head. This phenomenon, in itself, is undesirable because it interferes with signal detection from the recording layer of the media. If, in addition, the domain wall is mobile and its position is largely random at any given instant, unpredictable noise can be generated which significantly exacerbates the problem.
Magnetic anisotropy in the SUL is a key property affecting magnetic domain formation and structure. Magnetic anisotropy in soft magnetic materials is generally not intrinsic, but rather it can be controllably engineered by several means, including field-induced pair-ordering, deposition geometry, induced-shape anisotropy, and/or stress-induced magnetoelastic anisotropy. Techniques and materials have been developed to control SUL magnetic anisotropy such that no domain walls exist in the SUL layer of disk-shaped media, thus avoiding the above-mentioned problems. Radial and circumferential anisotropy distributions can be fabricated as to be domain-free (i.e., the SUL magnetic easy axis is or can be oriented in either the radial or circumferential direction). In practice, however, radial anisotropy of the SUL is most often employed in the manufacture of disk-shaped media since it occurs naturally when the SUL is deposited utilizing circular planar magnetron sputter deposition cathodes/targets in static, single-disk sputter deposition apparatus.
Regarding sputter deposition of magnetic materials, such as SUL materials, on disk-shaped substrates utilizing circular planar magnetron cathodes/targets (or any other type of vapor deposition source), it is noted that oblique vapor deposition of magnetic materials produces films with columnar grains that are magnetically oriented towards the vapor source. For disk-shaped perpendicular media, the orientation is radial, i.e., towards the center of the disk. When media are fabricated utilizing a static, single-disk sputter deposition apparatus with a circular-shaped planar magnetron cathode/target of a larger diameter than the outer diameter of the disk, the residual anisotropy is fortuitously predominantly radially oriented because most of the sputtered particle flux that arrives at the disk surface originates from the circular (“racetrack”)-shaped erosion region of the cathode/target surface located outside of the outer diameter of the disk.
By contrast, the conditions for obtainment of such radially oriented anisotropy of sputter-deposited magnetic layers on disk-shaped substrates are considerably less fortuitous when the media are fabricated in pass-by sputter deposition systems where the incident angle of the sputtered particle flux continuously changes as the disk travels through the deposition chamber and the resultant magnetic layer(s) has (have) little radial anisotropy. More specifically, when disk-shaped media are fabricated in batches (i.e., where a plurality of disks are mounted on a pallet and processed simultaneously) by means of pass-by sputter deposition systems, due to the pass-by motion and shape of the magnetron cathodes, the symmetry of the incident sputtered particle fluxes, as well as the symmetry of the magnetic field acting on the depositing SUL, are no longer radial. In fact, for elongated rectangular cathodes/targets typically used in pallet pass-by sputter tools, linear rather than radial anisotropy is typically induced, resulting in the formation of undesirable closure domains in the SUL.
In view of the foregoing, there exists a clear need for improved means and methodology for forming magnetic layers, particularly SUL films for disk-shaped perpendicular magnetic recording media, with substantially radially oriented magnetic anisotropy, utilizing cost-effective pass-by manufacturing technology and systems. Further, there exists a clear need for improved, disk-shaped perpendicular magnetic recording media which can be economically and cost-effectively fabricated utilizing pass-by manufacturing technology and systems.